The present invention relates generally to adjusting the air/fuel ratio in the cylinders of an internal combustion engine to control automotive emissions. More particularly, the present invention relates to a method and system for determining an oxidant set point location inside of a catalytic converter that is used in connection with a system that adjusts the air/fuel ratio in the cylinders based on the amount of oxidants stored in the catalytic converter.
To minimize the amount of emissions exhausted into the atmosphere, modern automotive vehicles generally include one or more catalytic converters, or emission control devices, in the exhaust system of the vehicle. These emission control devices store oxygen and NOx (collectively, oxidants) from the vehicle exhaust stream when the engine is operated with a relatively lean air/fuel ratio. On the other hand, when the engine is operated with a relatively rich air/fuel ratio, they release the stored oxygen and NOx, which then react with the HC and CO produced by the engine. In this way, the emission of both NOx and hydrocarbons (HC and CO) into the atmosphere is minimized.
The inventors have recognized a disadvantage with conventional air-fuel ratio control systems. In particular, the inventors have recognized that these systems attempt to maintain the engine at stoichiometry (or another desired air-fuel ratio). However, this has the disadvantage that engine air-fuel control is decoupled from the state of oxidant storage of the emission control device. The convention system relies on air-fuel feedback to compensate for this oversight.
To overcome disadvantages with prior approaches, the inventors have developed a method for controlling the engine air-fuel ratio to maintain the oxidant level stored in the emission system at a desired set-point level. However, the inventors have further recognized that there is another dimension to the emission system, namely the depth into catalyst along the direction of flow. I.e., oxidants are stored in different proportions depending on the depth, or length, of the catalyst brick. For example, at the front face of a catalyst brick, there is a high chance of oxidants being stored than at the back of the brick. As such, a control system that controls the engine based on the oxidants stored in the emission control device should consider where along the depth of the catalyst the set-point is chosen.
The above disadvantages are overcome by a method for controlling engine air-fuel ratio of an engine coupled to an emission control device having a predetermined length in a direction of exhaust flow. The method comprises:
selecting a position along the length of the emission control device; and
adjusting an operating parameter that affects engine air-fuel ratio based on an amount of oxidants stored in the emission control device and said position.
By selecting a location along the length of the catalyst for use in adjusting engine air-fuel ratio, it is possible to provide a minimum amount of oxidants storage reserve to account for oxidant storage control errors, while at the same compensating for changes in catalyst temperature and catalyst degradation.
In other words, oxidant storage capacity of a catalytic converter varies largely dependent on catalyst temperature and deterioration. Further, bricks in a multiple-brick catalytic converter tend to heat up (increasing their capacity) and deteriorate (decreasing their capacity) from the front of the catalytic converter to the rear. The inventive system for controlling engine air/fuel ratio recognizes that the available oxidant storage capacity of the catalyst is that which exists forward of the oxidant set point location. The present invention can then adjust the set point location in the catalyst based on the temperatures of the catalyst bricks, as well as their levels of deterioration. Specifically, the set point location starts out being positioned just behind the front-most brick in the catalyst. If the temperature of the second brick is sufficiently high to provide adequate oxidant storage capability, or if the deterioration level of the first brick exceeds a certain reference level, then the set point location is moved to just behind the second brick. This process is repeated for all of the bricks in the catalytic converter until a set point location is determined. The set point location also depends upon there being sufficient oxidant reserve capacity in the bricks positioned behind the set point.
Thus, by maintaining sufficient, but not excessive, reserve capacity behind the selected control position, improved performance results.
Note that the selection of the position along the catalyst length can be performed in various ways. The example outlined above is merely one example. Another example would be to select the position based on time since engine start. Further, a single location could be chosen for all vehicle operating conditions that compromises performance and simplicity in engine control.
Also note there are various ways to adjust an operating parameter that affects engine air-fuel ratio based on an amount of oxidants stored in the emission control device and said position. For example, fuel injection or engine airflow can be adjusted. As another example, engine breathing can be adjusted by changing engine valve timing. Further still, engine air-fuel ratio can be adjusted by adjusting fuel vapor purging, fuel injector pressure, or the number of fuel injections.